Binders or binder systems for foundry cores and molds are well known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder system. Typically, after the aggregate material and binder have been mixed, the resultant mixture is rammed, blown or otherwise formed to the desired shape or patterns, and then cured with the use of catalyst and/or heat to a solid, cured state.
In the foundry industry, the binder is typically from about 0.4 to about 6 percent by weight of the coated particle. Moreover, binder coated foundry particulates have a particle size in the range of USA Standard Testing screen numbers from 16 to about 270 (i.e., a screen opening of 0.0469 inch to 0.0021 inch).
Typically, the particulate substrates for foundry use are granular refractory aggregate Examples of refractory aggregates include silica sand, chromite sand, zircon sand, olivine sand and mixtures thereof. For purposes of the disclosure of the present invention such materials are referred to as "sand" or "foundry sand".
In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of aggregate material, such as foundry sand, and a binding amount of a binder or binder system. A number of binders or binder systems for foundry cores and molds are known. Typically, after the aggregate material and binder have been mixed, the resulting mixture is rammed, blown or otherwise formed to the desired shape or pattern, and then cured to a solid, cured state. A variety of processes have been developed in the foundry industry for forming and curing molds and cores.
One popular foundry process is known as the Croning or C process (more commonly known as the shell process). In this process, foundry sand is coated with a thermoplastic resin, a crosslinker and optionally other additives. Thermoplastic resin can be in solid form or in solution with a volatile organic solvent or mixtures of solvent and water. If the thermoplastic resin is a solid, the coating process requires the sand be heated to temperatures above the resin's melting point. Then the resin, crosslinker and other additives are coated evenly on the foundry sand to give a curable coating composition.
If the resin is in a solution, sand can be coated at temperatures at which the solvent can be readily removed. This process is also referred to as the liquid shell process. Frequently, crosslinker and additives are dissolved (or dispersed) in the solvent with the resin. The resinous mixture is added to warm sand. With agitation, the solvent is removed, leaving a curable coating on the sand particles. It is also possible to incorporate resin additives at other steps of the coating process.
In either case, a curable resin composition is coated onto the sand to form free flowing resin coated sand (particles). Subsequently, the resin coated sand is packed into a heated mold, usually at 350.degree. to 750.degree. F. to initiate curing of the thermoplastic polymer by reaction with the crosslinker to form thermosetting polymer. After the curing cycle, a shell of cured resin coated sand is formed adjacent to the heated surface. Depending upon the shape of the heated surfaces, shell molds and cores can be made and used in a foundry by this method.
Resin binders used in the production of foundry molds and cores are often cured at high temperatures, as discussed above, to achieve the fast-curing cycles required in foundries. However, in recent years, resin binders have been developed which cure at a low temperature, to avoid the need for high-temperature curing operations which have higher energy requirements and which often result in the production of undesirable fumes.
One group of processes which do not require heating to achieve curing of the resin binder are referred to as "cold-box" processes. In such processes, the binder components are coated on the aggregate material, such as sand, and the material is blown into a box of the desired shape. Curing of the binder is carried out by passing a gaseous catalyst at ambient temperatures through the molded resin-coated material. Where such processes use urethane binders, the binder components comprise a polyhydroxy component and a polyisocyanate component. These cure to form a polyurethane in the presence of a gaseous amine catalyst.
Another group of binder systems which do not require gassing or heating to bring out curing are known as "no-bake" systems. No-bake systems based on the use of urethane binders use an aggregate material, such as sand, coated with a polyhydroxy component and a polyisocyanate component. In this case, a liquid tertiary amine catalyst is combined with the polyhydroxy component at the time of mixing and the mixed aggregate and binder is allowed to cure in a pattern or core box at ambient or slightly higher temperatures.
As alluded to above, the binder for the urethane cold-box or no-bake systems is a two-part composition. Part one of the binder is a polyol (comprising preferably hydroxy containing phenol formaldehyde resin) and part two is an isocyanate (comprising preferably polyaryl polyisocyanates). Both parts are in a liquid form and are generally used in combination with organic solvents. To form the binder and thus, the foundry sand mixture, the polyol part and the isocyanate part are combined. After a uniform mixture of the boundary sand and parts one and two is achieved, the foundry mix is formed or shaped as desired. Parts one and/or two may contain additional components such as, for example, mold release agents, plasticizers, inhibitors, etc.
Liquid amine catalysts and metallic catalysts, known in the urethane technology, are employed in a no-bake composition. The catalyst may be incorporated into either part one or two of the system or it may be added after uniform mixing as a part three. Conditions of the core making process, for example, work time (assembling and admixing components and charging the admixture to a mold) and strip time (removing the molded core from the mold) can be adjusted by selection of a proper catalyst.
In cold-box technology, the curing step is accomplished by suspending a tertiary amine catalyst in an inert gas stream and passing the gas stream containing the tertiary amine, under sufficient pressure to penetrate the molded shape until the resin is cured.
Improvements in resinous binder systems which can be processed according to the cold-box or no-bake process generally arise by modifying the resin components, i.e., either the polyol part or the isocyanate part. For instance, U.S. Pat. No. 4,546,124, which is incorporated herein by reference, describes an alkoxy modified phenolic resin as the polyhydroxy component. The modified phenolic resin improves the hot strength of the binder systems. U.S. Pat. No. 5,189,079, which is herein incorporated by reference, discloses the use of a modified resole resin. These resins are desired because they emit reduced amounts of formaldehyde. U.S. Pat. No. 4,293,480, herein incorporated by reference, relates to improvements in the isocyanate component which enhances shake-out properties of non-ferrous castings.
Epoxy resins have been used in the formulation of phenolic foundry binders. For example, Plastiflake.RTM. 1114 and Plastiflake.RTM. 1119 novolac resins (which are not urethane resins) contain epoxy resins as plasticizers as disclosed by U.S. Pat. No. 4,113,916 to Craig, incorporated herein by reference. Kerosine, a mixture of aliphatic and aromatic hydrocarbon, has been employed in urethane binder formulations. Kerosine is a common solvent found in urethane binders. However, the known uses of kerosine in urethane do not include epoxy.
Water based coatings are often employed with resin coated foundry sand. The coatings are employed to make the mold or core more resistant to heat or to provide molds and cores having improved surface characteristics. However, the water based coatings can degrade the urethane coating on the foundry sand. It would be advantageous to provide an additive for urethane resins for foundry use which is highly resistant to water based coatings. Also, conventional urethane coatings and molds or cores lose strength during heating. It would be desirable to achieve improved resistance to losing strength during heating.